1. Field of the Invention
Water-absorbent resins based on acrylic acid, in particular those known as superabsorbents, are widely used in, for example, hygiene articles.
2. Description of the Background
It is known that acrylic acid can be prepared by heterogeneously catalyzed gas-phase oxidation of propane, propylenene (and/or acrolein) by means of molecular oxygen over solid catalysts. The gas-phase oxidation of propane, propylene and/or acrolein forms the desired acrylic acid together with by-products, in particular in the form of carbonyl compounds, e.g. aldehydes such as benzaldehyde, furfurals, propionaldehyde etc., and also formic acid and maleic acid or maleic anhydride, whose presence has an adverse effect on the polymerization if the acrylic acid is used for preparing superabsorbents. Aldehydes hinder the polymerization of the acrylic acid and also lead to discolored polymers. The presence of maleic anhydride in the polymerization leads to formation of undesired copolymers which influence the properties of the desired polymers. Contamination by carboxylic acids which are not capable of polymerization, e.g. formic acid, are particularly disadvantageous when the polyacrylic acids prepared by polymerization are used in hygiene articles which come into contact with human skin, since these carboxylic acids cause extreme skin irritation. The presence of oligomeric acrylic acids, e.g. diacrylic, triacryilic or tetraacrylic acid, is troublesome because the oligomeric acrylic acid present dissociates during the thermal treatment of the polyacrylic acid and liberates acrylic acid, which likewise causes extreme skin irritation.
In the preparation and/or isolation of acrylic acid, use is generally made of process polymerization inhibitors such as phenothiazine, hydroquinone or hydroquinone monomethyl ether to suppress undesirable premature polymerization, particularly at elevated temperatures. Undesired polymer formation leads to deposits on heat exchanger surfaces and column trays and also to blockage of lines, pumps, valves etc. Since the process polymerization inhibitors, which have an excellent inhibiting action, naturally also retard intended preparation of polyacrylic acid, they subsequently have to be removed again from the acrylic acid obtained and be replaced by storage polymerization inhibitors which have less of an inhibiting action.
To remove or reduce the amounts of the abovementioned by-products and impurities, multistage distillation and/or extraction and/or crystallization steps are generally employed in the prior art. Acrylic acid which is obtainable in this way and is suitable for preparing absorbent resins is generally referred to as pure acrylic acid. Thus, EP 0 754 671 discloses a distillation process for purifying acrylic acid prepared by catalytic oxidation of propylene, in particular to remove maleic anhydride. EP 0 727 408 discloses a two-stage distillation of crude acrylic acid. The fraction taken off at the top, which comprises formic acid and acetic acid, is esterified to make use of the acetic acid.
DE 2 241 714 discloses a process for separating acrylic acid from the reaction gases from the oxidation of propylene or acrolein by means of countercurrent absorption, with acetic acid and some water being stripped from the resulting absorption solution by means of inert gases. DE 4 308 087 describes a process for separating acrylic acid from the reaction gases from the catalytic oxidation of propylene and/or acrolein by countercurrent absorption using a mixture of diphenyl ether, biphenyl and dimethyl o-phthalate.
DE 196 34 614 discloses a process for separating off pure (meth)acrylic acid by distillation in a distillation apparatus which comprises a thin film evaporator, a condenser and a connection comprising an impingement device between thin film evaporator and condenser.